1. Field of the Invention
This invention relates to an electric power supply unit for providing a predetermined output voltage through conversion of a power supply voltage of a dc power supply such as a battery, and to a portable device equipped with such power supply unit.
2. Description of the Related Art
A power supply unit such as a series regulator typically converts the power supply voltage of a dc power supply to a predetermined output voltage. This power supply unit is required to provide a stable output voltage which is free of ripples that arises from ripple components in the power supply voltage.
This power supply unit is controlled such that the feedback voltage associated with the output voltage is brought to a given reference voltage. Therefore, in order to stabilize the reference voltage, it is inevitable to reject ripple components in the output voltage (referred to as ripple rejection), since the output voltage greatly contributes to the stability of the reference voltage.
The reference voltage is usually generated from the power supply voltage supplied to the power supply unit by a reference voltage generation circuit. As a consequence, the reference voltage contains ripples, which makes it difficult to perform sufficient ripple rejection.
In order to solve this problem, a voltage adjustment circuit (hereinafter referred to as pre-power supply circuit) is provided for stabilizing the power supply voltage. The output voltage of the pre-power supply circuit is supplied as the operating voltage of the reference voltage generation circuit. Japanese Patent No. 2001-84043 (referred to as Patent Document 1) proposes use of a pre-power supply circuit to improve the ripple rejection characteristic of a power supply unit against ripples in the power supply voltage.
However, provision of a pre-power supply circuit entails current consumption thereof, which increases the overall current consumption. A portable device that uses a battery as the power supply particularly requires minimization of current consumption to ensure prolonged use of the battery. From this point, it is not desirable to provide a current consuming pre-power supply circuit.
Moreover, a pre-power supply circuit lowers the power supply voltage a little due to its power consumption. For this reason, the power supply voltage for generating a required reference voltage must be higher in the case where a pre-power circuit is used than in the case where no pre-power circuit is used. That is, the reduced-voltage characteristic of the power supply unit is degenerated. As a consequence, a problem arises then that the minimum permissible level of the power supply voltage of the battery is raised, and accordingly the usable time of the battery is shortened.